I work on two computers with one USB headset. I want to listen to both by piping the non-Linux computers' output into the Linux computer's line in (blue audio jack) and mixing the signal into the Linux computer's headset output using PulseAudio.

Please edit this so it makes sense even if the other answer is deleted. Reference the other answer (by stating the name of the author), but copy all essential information into your answer.
– G-ManDec 1 '18 at 18:50